Prostate cancer (PCa) is the most common non-cutaneous cancer in men in the United States, with over 185,000 cases newly diagnosed, and over 28,000 deaths annually [1, 2]. Screening methods are now widely used in the United States and Europe to detect PCa, which include digital rectal examination (DRE), and prostate-specific antigen (PSA) analysis. When suspicion is raised through these screening mechanisms, prostate biopsies are performed to diagnose PCa, which depends upon the presence of adenocarcinoma in biopsy cores, with clinical significance being determined by the presence of 50% or more PCa tumor tissue in 3 or more biopsy cores, a Gleason sum (GS, histologic analysis) greater than 6, and a PSA density (PSA divided by ultrasound derived volume of prostate) more than 0.15[3]. The clinical standard for performing prostate biopsy is ultrasound-guided, transrectal, laterally directed 18G needle cores, with the number of cores ranging from 6-12, systematically sampling different regions of the prostate[4]. This standard does not involve targeting needles to suspicious regions since PCa does not have unique B-mode ultrasound image characteristics that can delineate diseased from normal structures and benign pathologies of the prostate (e.g., benign prostatic hyperplasia (BPH) and inflammation). The current standard of care has a dismal sensitivity (only 53% using octant systematic biopsy in a cohort of radical prostatectomy specimens with previously diagnosed, clinically significant disease, [5]), mainly because the sampling grid only randomly intersects the pathologic tissues. Over 1,000,000 prostate biopsies are performed annually in the United States[6], with PCa detection rates being low (25-36%)[6]. PCa detection rates on repeat biopsies (cases with negative first biopsies) are again 10-35% [7, 8]. The fact that these rates are identical suggests that as many cancers are detected as are missed during a single systematic biopsy session. In addition, many of the cancers that are detected with this approach are clinically insignificant[3]. Elastography imaging methods have shown promise for PCa visualization and prostate biopsy guidance based upon stiffness differences between normal and pathologic tissues[9]. However, elastography methods can be limited by an inability to apply uniform compression (stress) to the prostate using a hand-held transrectal ultrasonic transducer. Acoustic Radiation Force Impulse (ARFI) imaging is an elastography imaging method that we have developed at Duke University that overcomes these challenges through the use of focused acoustic beams for the application of stress. We have obtained promising initial ex vivo and in vivo results, in which normal prostatic structures and focal PCa lesions are clearly visualized in ARFI images that are not visualized in matched B-mode images. The in vivo data demonstrate a clear advantage of ARFI imaging over conventional elastography for PCa visualization, in that the acoustic energy is coupled directly into the prostate. We propose to develop and optimize dedicated 2D and 3D transrectal in vivo ARFI imaging methods for the purpose of visualizing PCa and differentiating PCa from benign processes in the prostate; to evaluate the correlation between suspicious regions in ARFI images and tissue histology, to investigate the correlation between local PCa Gleason pattern (measure of histologic aggressiveness) and PCa visibility in ARFI images, and to evaluate the potential increase in biopsy detection rate of clinically significant PCa under ARFI image guidance. If successful, this research has the potential to greatly improve cancer detection rates for clinically significant grade PCa (i.e. GS 7 or more) during first time biopsies, to reduce the number of biopsy cores taken during biopsy, to facilitate longitudinal monitoring of PCa growth or recurrence after in situ therapies, and to provide image guidance for focal PCa therapies.